Is the Riddler really some kind of watered down Joker? I would say: No.
Of course in "Forever" he is. But in this movie even Two-Face is a cheap Joker rip-off.
The Riddler can be shown and written as a very intelligent and sane guy who is absolutely not random at all. The problem is that Nolan's Joker was absolutely out of character. Nolan's Joker was a schemer and IMHO he was not crazy. Not at all. So maybe there are no huge differences between his version of the Joker and his version of the Riddler. But in the comic, there are.